OBJECTIVE To investigate the importance of small (SK)- and intermediate (IK)-conductance Ca2+-activated K+ channels on bladder function, by studying the effects of 4,5-dichloro-1,3-diethyl-1,3-dihydro- benzoimidazol-2-one (NS4591), a new modulator of SK/IK channels, on contractions induced by electrical field stimulation (EFS) and carbachol in rat, pig and human detrusor. PATIENTS AND METHODS Detrusor biopsies were obtained from rats, pigs and male patients undergoing cystectomy because of bladder cancer. Force was recorded using myographs. Intracellular free Ca2+ was measured in myocytes using microfluorimetry. RESULTS In rat bladder rings subjected to EFS, cumulative addition of NS4591 (0.1-30 μM) decreased force by 82 ± 2.9% (n = 6).This effect was reduced by 64 ± 5.2% in the presence of 0.3 μM apamin, a specific inhibitor of SK channels. Apamin increased the force evoked by EFS significantly: force was increased by 14.2 ± 3.4% (n = 5) and 10.1 ± 2.6% (n = 7) in pig and human detrusor strips, respectively (P = 0.04 and P = 0.02). The cumulative addition of NS4591 (0.3-30 μM) significantly reduced the amplitude of carbachol-induced rhythmic oscillations by 62.0 ± 12.0% (n = 12) and the minimum force between oscillations by 30 ± 5% (n = 9) in pig detrusor strips (P < 0.005). In the presence of 10 μM NS4591, carbachol (1 μM) induced rhythmic contractions with an amplitude and normalized mean power frequency (nmeanPF) of 8.4 ± 5.1% and 0.11 ± 0.06 mN root mean square (rms) Hz (n = 12), respectively, vs. 21 ± 3.4% and 0.17 ± 0.04 mN rms Hz in control strips (n = 13). Apamin induced 6- and 11-fold increases in amplitude and nmeanPF vs. 1.3- and 2-fold increases in control strips. In human detrusor strips (n = 15), the cumulative addition of NS4591 (1-30 μM) significantly reduced the amplitude by 69 ± 11%, the nmeanPF by 78 ± 6% and the minimum force between carbachol-induced oscillations by 59 ± 5% (P < 0.008). The addition of apamin (0.3 μM) before application of 1 μM carbachol abolished the effects of NS4591 on amplitude and partially abolished its effect on nmeanPF by 41 ± 7%, vs. a 78 ± 6% reduction in the absence of apamin (n = 8). In spontaneously active detrusor preparations, NS4591 reduced or abolished contractions. Furthermore, NS4591 (10 μM) decreased the carbachol-induced increase in the fura-2 ratio by 43 ± 3% compared with control (n = 12) (P < 0.03). CONCLUSIONS The SK/IK channel modulator NS4591 inhibits EFS- and carbachol-induced contractions in rat, pig and human detrusor muscle. NS4591 may have therapeutic potential for treatment of detrusor overactivity.